A background summarization of the utilization of wind for generating electrical power has been documented in a NASA publication entitled “The Development of Wind Powered Installations For Electrical Power Generation In Germany”, U. Hutter, NASA TTF-15,050 National Aeronautical and Space Administration, Washington, D.C. 20546, August 1973. Suffice it to say, one of the oldest methods of extracting energy from fluids in motion is by means of bladed rotating machines such as, for example, windmills exposed to the wind and hydro impellers powered by the fluid reaction of moving water. A history of the development of wind energy is presented by the publication entitled, “Wind Energy Developments In The Twentieth Century”, published by the Lewis Research Center of the National Aeronautics and Space Administration, Cleveland, Ohio, in 1979. A wind turbine system, termed the “Mod-2 Wind Turbine”, is described both in that publication and in another publication simply entitled, “Mod-2”, published by the Bonneville Power Administration, U.S. Department of Energy. While these devices have been successfully employed for many hundreds of years, there still remain major unresolved technical problems in the design of conventional wind turbines which make them problematic for small and large scale energy production.
These technical problems fall into basically three categories: fluid dynamics, dynamic stress, and electrical conversion.
The fluid dynamic difficulties can best be appreciated by the Betz theory which expresses limitation on potential energy conversion and which is described in Technical Note #75 “Meteorological Aspects of Utilization of Wind as an Energy Source,” World Meteorological Organization 1981. The column of air (wind) impelling upon the windmill blades is slowed and its boundary is an expanding envelope where streamlines meet turbulence behind the rotor. Attempts to shroud the envelope to utilize the lower pressure behind the blades as a fluid dynamic advantage have been suggested in U.S. Pat. No. 4,075,500 as well as others. To date no practical cost-effective method exists for fabricating ducted shrouds, much less balance them on support structures.
Mechanical stresses induced on the blading and support structure present further limitations especially for large windmills. On the supporting structure the axial stress representing the force which tends to overturn the windmill, or the thrust on the bearing, must be kept within limits at all wind speeds. To accomplish these results and to generate sufficient and efficient power, large diameter blades with built-in governors for adjusting the pitch angles of the blades have been utilized. These mechanisms proposed to date make the blades fragile and costly.
Furthermore, large diameter blades, such as over 100 feet in length, present significant dynamic stress problems. The increased blade length of larger rotors requires greatly increased blade stiffness and reduced weight in order to insure that critical vibrational frequencies of the blade remain sufficiently outside the excitation frequencies associated with routine operation so that the blades do not become unstable. A combination of gravitational force and torque force on each blade element functions to cyclically stress the blade element as it rotates in a rising direction and then a falling direction. Long blades supported at their roots and under the influence of the aforementioned oscillating forces are subjected to an increasingly severe and complex system of dynamic instabilities. It is difficult and expensive to safeguard against such instabilities. Blade stiffness to weight ratio improvements and advanced design methods can help but there is always a practical maximum to the size of blades being employed by wind turbines.
Finally, conventional wind turbines generate power through a gearbox and conventional generator. Although these generators are commonly available, they represent a one hundred year old technology that has not incorporated recent technological breakthroughs in material science, power control and aerodynamics. These generators, while compact, are heavy because of the massive laminated iron core and copper windings.
It is well known now that the change of wind momentum by aerodynamic forces on the rotor blades is limited in energy output by several factors. First, the ideal efficiency is limited to about 0.593 of the wind power per unit area by the unavoidable shedding of flow around the tip of the rotor. Secondly, occasional extremes of wind speeds can lead to destruction of the rotor blades, unless controls be employed to reduce the geometric obstruction presented by the rotor. This control measure limits maximum output to be expected from a wind turbine. Thirdly, the aerodynamics of the tower supporting the wind driven rotor must be taken into account in the design of the blades whether located ahead of or behind the rotor to the determent of structural cost to accommodate the aerodynamic loading such structures create.
It has long been thought that substantial performance advantages could be realized by the use of a shroud and diffuser on a wind turbine (see “A Preliminary Report on Design and Performance of Ducted Windmills”, G. N. M. Lilley and W. J. Rainbird, the British Electrical and Allied Industries Research Association, Great Britian, Technical Report C/T119, 1957). Prior work indicated that the diffuser augmented turbines could produce up to twice the power of unshrouded turbines of the same diameter. Thus, over the years, many groups have become interested in the diffuser-augmented windmills, but they have always dropped the idea because the diffuser has to be so much larger than the windmill rotor that the system has been heretofore considered economically impractical.
An electric generator, driven by a wind turbine serves as a source of electric power which may be utilized at remote sites where electric power may not normally be available. For example, in a sailboat moored, anchored or at a dock between usage periods, such a turbine driven generator may be deployed for recharging the batteries which power the radios, pumps, lights, and other electrical devices aboard the sailboat. Such devices can also be used on land vehicles as well.
As can be understood from the foregoing, the windmill has developed over its long history into many different and varied forms. These varied forms seek to expound upon particular aerodynamic or economic factors. In some cases, prior art windmills have been designed for aerodynamic efficiency in low speed winds. Such high efficiency windmills often were prone to damage or destruction from excessive rotational speed when exposed to high speed winds. Since the velocity of winds vary over a wide range in most areas, aerodynamic efficiency is not the sole factor for providing an economically attractive windmill.
All windmills suffer from the large forces developed on the windmill and its tower or other supporting structure during times of high velocity winds. These large forces require that both the windmill and supporting structure be strongly built. Windmills are usually mounted atop relatively tall towers to take advantage of the higher velocity winds which exist only a short distance above the ground. High wind loads applied to the windmill and tower create very large bending moments which the tower must support. In the case of horizontal axis windmills, these large bending moments cannot be alleviated by the use of guy wires or struts because the rotating windmill blades pass near to the tower along all sides of the tower as the direction of the winds change and the windmill turns with the wind. This dictates that long, slender towers without guy wires be used. Such towers are relatively expensive and the overall cost of the windmill installation is increased when compared to towers supported by guy wires.
Large scale prior art horizontal windmills also cause additional forces to be applied to the tower when the large rotor blades pass very near the windmill tower. In such large windmills, the periodic passage of the blades by the tower can create a vibratory loading which further accentuates the problem of building a sufficiently strong tower.
Another problem suffered by prior art vertical and horizontal windmills is the large fluctuations in windmill rotor speed caused by varying wind velocity. This variation in the rotor speed creates difficulties in coordinating power generated by windmills into an electrical distribution system having a fixed frequency of alternating current. The great fluctuation in windmill speed also creates problems in the mechanical design of windmills since stresses increase considerably with increased rotational speed.
Sudden changes in wind direction also have been a problem in obtaining optimum efficiency from windmills. This is especially true with horizontal axis windmills which are resistant to changes in wind direction because of the gyroscopic effect of the spinning windmill. This gyroscopic effect reduces the efficiency of the windmill because it cannot change direction sufficiently fast so as to take advantage of the full force of the wind. Vertical axis windmills have been effective in dealing with the wind direction change and gyroscopic effect problems but have still suffered from wide fluctuations in angular speed due to wind velocity changes. Sudden changes in wind direction can also cause the rotor blades to warp or flap and in some cases damage or destroy them.
Therefore, there is a need for an improved windmill that is efficient, yet is not as subject to damage from high and changeable winds as prior art windmills.